The delta H of formation of FeBr3 (s) is 269 kJ/mol and the delta H for the reaction 2Fe (s) +3Br2 (g) –> 2FeBr3 (s) is 631 kJ/mol. Calculate the delta H of evaporation of Br2 (l) which is a liquid at 25 degrees C
That answer isn’t correct :/
2 Answers

The equation for the formation of FeBr3(s) is:
Fe(s) + 3/2 Br2(l) –> FeBr3(s) DH = 269 kJ
For your second equation, you are given:
2 Fe(s) + 3 Br2(g) –> 2 FeBr3(s) DH = 631 kJ
You are looking for:
Br2(l) –> Br2(g) and its DH.
So, you should halve the second equation and flip it over. That changes the sign of DH and give you:
FeBr3(s) –> Fe(s) + 3/2 Br2(g) DH = +315.5 kJ
Now, if you just add that to the first equation and cancel everything that appears on both sides, you are left with your desired equation. When you add the two DH values you have:
Br2(l) –> Br2(g) DH = +47 kJ

31.0 kJ/mol
Source(s): mastering chemistry